So you can sign up for a specific instance of lemmy or kbin, and then see and interact with any content from any other instance of the Fediverse, provided they have not defederated. Where things get a little weird is how upcoming apps will function. I was told that Sync for Lemmy will likely require you to have a Lemmy account to sign in, but will still allow you to browse and post to any Fediverse content. This may end up being just a short-term issue until developers can "line up API endpoints", whatever the Treknobabble that means.